HUNT FOR DNA AMPLIFIED IN CANCERS UNCOVERS IMPORTANT TARGET
GENE

Boston,
MA-August 7,2006-Researchers at Harvard Medical School (HMS) and Massachusetts
General Hospital (MGH) have discovered a new cancer-promoting role for a
gene potentially involved in breast, liver, and other kinds of cancers.
Their discovery that the gene YAP can transform mammary epithelial cells
opens the door to understanding how a novel cell growth controlling pathway
first discovered in fruit flies might be important in human cancers. This
work is published in the Aug. 8 online early edition of the Proceedings of
the National Academy of Sciences and will appear in the Aug. 15 print
edition.

“We screened the DNA from breast cancer cells for amplifications that are
associated with tumor development. The identification of these new potential
cancer-causing genes is critical to uncovering novel pathways that drive the
conversion of a normal cell to a cancerous one.” says senior author Daniel
Haber, MD, PhD, the Laurel Schwartz professor of medicine at
HMS and MGH and director of the MGH Cancer Center (top picture). This research
was
conducted
jointly by Haber’s lab and the lab of Joan Brugge, PhD, professor
and chair
of the Department of Cell Biology at HMS.

Through microarray analysis of a mammary tumor in a BRCA1/p53 deficient
mouse model, Haber’s group discovered an amplified region of DNA in the
mouse breast tumor that contained only one known gene, called YAP.

“A similar region of DNA is also amplified in some human tumors, but this
amplified region often contains other genes that are known to promote cell
survival,” says Haber, who worked with co-authors Jianmin Zhang, PhD, and
Gromoslaw Smolen, PhD, both research fellows at MGH. “Thus, whether the YAP
gene could play a role in these cancers had been largely ignored. The
amplified region we discovered excluded these other genes, which allowed us
to focus on YAP as a new candidate.”

The YAP gene has an interesting literature associated with it that comes
from the fruit fly Drosophila melanogaster. The Drosophila version of the
YAP gene, called Yorkie (Yki), functions to promote both cell division and
cell survival and is controlled by several other genes called Hippo (Hpo),
Salvador (Sav), Warts (Wts), and Mats. The mutation of any of these upstream
genes or the overexpression of Yki causes dramatic overgrowth of cells in
the Drosophila eye or wing. This coupling of cell division and cell survival
is unique ­ other genes that promote cell division, for example, Myc, also
sensitize a cell to death.

“To use the car analogy that is often applied to cancer models, activation
of Myc is like stepping on the gas to activate cell division but also
lightly tapping on the brakes at the same time, so that should anything go
wrong during division, the car can very quickly be stopped, or the cell can
be removed by cell death,” says first author Michael Overholtzer, PhD,
research fellow in cell biology at HMS. “Yki activation, on the other hand,
is like stepping on the gas and disabling the brakes at the same time. Such
an activity would be thought to be coveted by cancer cells. Therefore these
genes, Yki (YAP), Hpo, Sav, Wts, and Mats, most of which were first
discovered in the fruit fly, represent a relatively new and exciting pathway
that might control human cancers.”

Earlier studies on YAP function in human cells did not support the notion
that YAP might be a cancer causing gene because its overexpression actually
promoted cell death rather than cell survival (like Yki in Drosophila).
Nevertheless, due to the amplification of YAP in a mouse breast tumor,
Overholtzer and colleagues decided to examine the functions of YAP in a 3D
mammary culture model developed in Brugge’s lab.

In this model, they grew cells in a 3-dimensional protein matrix rather than
in 2-dimensions on plastic, which allows mammary cells to adopt an
architecture in culture that is similar to what occurs in the human breast.
They had previously uncovered the effects of other genes using this model
that would be missed in more conventional 2D models.

Using these 3D cultures, the authors were able to show that the
overexpression of YAP caused a dramatic change in cell behavior associated
with invasion into the protein matrix. This type of invasive activity is
normally associated with strong acting cancer-promoting genes. The authors
were further able to show, in 3D cultures and other assays, that YAP
overexpression both activated cell growth and inhibited cell death, just as
one might have predicted from the studies of Yki in Drosophila.

Moreover, YAP overexpression was able to turn their non-cancerous mammary
cells into cancer-like cells in the lab, as evidenced by the ability of YAP
expressing cells to grow in soft agar, an assay that measures cancerous
potential. Parallel to Overholtzer and colleague’s work, the lab of Scott
Lowe, PhD, of Cold Spring Harbor, also showed that YAP overexpression could
contribute to the development of liver tumors in a mouse model (Cancer Cell,
July 2006). Thus, it appears that YAP is indeed a newly identified
cancer-causing gene.

“What we would like to understand next is how YAP is controlled by the
Hpo-Sav-Wts pathway in human cells”, says Overholtzer. “Also, although we
found the YAP amplification in a mouse breast tumor, in human cancers this
amplicon is actually much more common in other types such as lung,
pancreatic, ovarian, and others. Thus it is possible that YAP plays an
important role in the development of many different types of cancer.”

This work was supported by the National Institutes of Health, the National
Cancer Institute, the Breast Cancer Research Foundation, the Doris Duke
Foundation Distinguished Clinical Investigator Award, and the National
Foundation for Cancer Research.

MASSACHUSETTS GENERAL HOSPITAL, established
in 1811, is the original and largest teaching hospital of Harvard Medical School.
The MGH conducts the largest hospital-based
research program in the United States, with an annual research budget of nearly
$500 million and major research centers in AIDS, cardiovascular research, cancer,
computational and integrative biology, cutaneous biology, human genetics, medical
imaging, neurodegenerative disorders, regenerative medicine, transplantation
biology and photomedicine. MGH and Brigham and Women's Hospital are founding
members of Partners HealthCare HealthCare System, a Boston-based integrated
health care delivery system.